We've seen set images of Jennifer Lawrence as Raven Darkholme a.k.a. Mystique with James McAvoy as Charles Xavier, in Matthew Vaughn's X-Men: First Class. But other than the the slick 1960's look, we don't really have any idea what to expect to see from the characters in the film.
In a recent interview with Jeff Wells, Lawrence talks about her character Mystique in the film and what we can expect to see. This character was previously played by Rebecca Romijn. Here's Well's description of what she said,
Only 10% of her screen time will be as Mystique, she said. It takes X-Men makeup artists about six hours to transform her into Mystique with the blue skins and the scales, she added. (She has a nickname for the process or the makeup or whatever — “Mystink.”) I asked if official photos have been taken, and she said the idea was to keep her appearance under wraps until just before the film’s release.
So it looks like Mystique's look is going to be somewhat similar to what we've already seen. I'm sure they have made a few changes to it to correspond with the 1960's retro feel that the film will have. I also imagine that we will only get to see around 10% of most of the X-Men characters as their mutant selves. After all, this is an origins film.