I agree with Undead Master. Robert Riskin deserves a lot of credit for scripting his best movies and indeed it was a great partnership. It seems just as fair to give Capra's actors/actresses, especially Stewart's two towering performances, a lot of credit as well. (How did Stewart not win for Mr. Smith?) Throw in all those great supporting charactsrs that endlessly spiced up all 30's screwball comedies and Sturges movies?
But overall, Capra's movies have a great vision of positive attitude during an individual darkest times. They appear very seamlessly made but there was a lot of hard work and talent to make such beloved films. Without a great script it is hard to make great movies but a lesser director or even one who does share Riskin's outlook on life would have delivered lesser movies.

There is a tendency to favor directors over writers when it comes to giving credit, and i think that's probably justified.
In this case, it's obvious that Capra had some great scripts to work with, but if you handed them to somebody else, I doubt the resulting movies would have felt like frank capra movies. They may have been good, or they may have been mishandled, but i don't think anybody would have mistaken them for Capra's films. I don't really think the "Capra Touch" was only in the themes, but more about the way they were handled, dramatized and filmed.
When a director makes a movie, he puts a little bit of himself into it. Every little choice he makes is a reflection of his inner values and his ideas. These things are what make movies special/personal and they aren't contained in a script. Capra's movies, in particular, have a very "personal" feeling. If you watch enough of them, you almost feel like you know him.

In the beginning was the word. It all starts with the story, the script, the play. That said, it takes someone with the heart and soul of Frank Capra to present the audience with a great movie.

Ce la Vie.
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