its going to rain and its going to be cold.
Everybody can shoot great pictures if the conditions are easy. Making it work no matter what is part of being professional.
I think that the effort the photographer has to put into his work grow exponentially the further away the conditions are from ideal. If it rains for them and is cold, be mentally prepared that for you this can mean being wet to the skin and chilled to the bone. I'm not saying that this necessarily happens. The point is that you should always stand above the situation and never let yourself go to the train of thought that the situation sucks. Regardless if it does or not.
And I don't mean that in a cynical or sarcastic, but rather a stoic way. You will face those moments when you see their smiles drop. Don't encourage that with something like "well, we all know it's not great, but could you please give me a smile for the image?".
Instead embrace the reason for all that in a positive way "hey look at that handsome guy who's holding you in his arms." or "aaw, you look so cute together, I'm almost jealous".
The best mental preparation might be to watch Apocalyse Now Redux and keep a positive mindset throughout the entire film.
It's more or less the opposite of what the couple wants and they are going to be pissed.
That doesn't come as a surprise. But then I never really understood the whole "best day of my life"-thing. Isn't it somewhat insulting to make a decision to spend a life together with somebody else and at the same moment claiming the last day one is not married to that person is the "best day ever"?
"Hey you, I want to be together with you for the rest of my life, but as the wedding day is the last day that I will not be married to you for the entire day, I declare this to be the best day of my life."
I'm exaggerating. (can't you tell?) But at the essence the thought is valid, the whole thing is only partially about having a great day, but mostly about being together forever and always and that kind of jazz. (that bold word right there, that's the 421 for wedding questions)
Your best bet is to exploit exactly that. Make two-people-being-together the main subject, if that happens to be on judgement day, so be it.
I'm lamenting about the mental and emotional aspects of this so extensively because they are very important for a wedding, if not most important.
A few ideas for applications of these ideas below. It really depends on the people, the wedding etc. if they are applicable.
humorous takes on "classical" settings
Say Cinderella for example. Sitting bride, kneeing groom, trying to fit a shoe to her foot. What if the shoe is a rubber boot? There are very colorful rubber boots available, making for great subjects.
Imagine you flick through a wedding album and you see a rubber boot. What's the story behind that? It's a different take on the cute pose that you see in every other wedding album. There's a bit of a story behind it. It's also a play on words "going through" bad times together.
together in the rain
This may or may not be possible. Talk to the bride and groom to what extent it is acceptable for them to get wet and adjust to that.
- Bride and groom together under the jacket of the groom protected from the rain. Depending on the lighting, add a small warm light source (candle?) to illuminate them.
- Either bride or groom holding only a very small umbrella (almost comically, maybe a newspaper as a makeshift umbrella), that is barely large enough protect her/his head from the rain. Next to him is the other partner with a large umbrella big enough for both of them. With eye contact and a bit of gesturing, that's quite a story to tell with one image.
- Close friends of them holding something that protect bride and groom from the rain while the others get soaking wet. Depending on how important friends and family are to the couple, this can be an important image for them. You could try this without the other people getting wet too much, but I doubt it would sell the effect. Talk to some of the guests beforehand if they would do this and prepare with towels and additional cloths so they can change afterwards. Bonus points for not telling the couple about it, seeing them surprised about what their friends/family would do for them getting entirely wet. No chance of getting fake looks this way.
Can you get umbrellas or rain coats in different vivid colors? That would make for an interesting group shot outside.
On crowded sidewalks in rainy weather, there's almost a closed "roof" of umbrellas above peoples heads. Can you fill the frame with colorful umbrellas and - oh! that one umbrella being tilted up a little, unveiling the kissing couple underneath?
time management and assistants
Don't do everything at once. Keep a few cards up your sleeve so you can pull them when the mood is going down. "I have this other idea for a photo..." might be the reoccurring catchphrase of yours that saves the day. Especially if it's always a little surprise for the bride or groom or both what's going to happen then and if they liked the previous shots.
Get assistants to help you either hired ones or guests (talk to them beforehand). Pulling off a photo shooting in the rain should not take long. Everything should run smooth and if there's a setup required (like for the umbrella shot) it's best to have everybody know what to do (except for bride and groom if it's a surprise).
If the bride is saying "I'm not stepping out there in only my dress for longer than 3 minutes". Then you want to smile and say "We'll be done in 2"2 then with the help of your assistants get the shot quickly. During the time it takes to dial in settings or explain the idea to a lot of people while they are in the rain and/or cold will build up grumpy feelings. Avoid that at any cost with preparation. It should work like clockwork, which is part of the experience you sell here: everything works. We are prepared for everything. No, the rain doesn't matter. * click * "Hey look at this image of you and your friends! Like it? Great, let's get back inside again."
going to 11
Photographically speaking, rain is awesome at night. The entire wet ground is a reflecting source, multiplying every light source in the scene. Even a regular street with some street lights can turn into a great looking scenery. Place the couple in that scenery and add a flash if necessary to get a great looking image.
Embracing the rain as a tool can yield results that cannot be obtained on a sunny day.
If the couple concludes at the end of the day that "The others may have sunny weather, but we have better photos" despite being tired, cold and wet, then you accomplished your goal.
I excluded any suggestions for indoor photography. I'm sure you have plenty thereof.
If you need something for the guest book or album:
Life isn't about waiting for the storm to pass...It's about learning to dance in the rain. - Vivian Greene
1which is the Answer to the Ultimate Question of Life, The Universe, and Everything