To what extent were the actors in Wedding Crashers improvising?

According to the trivia on IMDb, Owen Wilson and Vince Vaughn improvised the "Lock it up!" banter. As I understand it, that also means the other scenes did not - or only slightly - diverge from the script.

@LaurentPireyn highlights a good example of their improvisation in the "lock it up!" banter over on IMDb, but there is more out there.

There are a number of sources in which Vince Vaughn and Owen Wilson speak to their methods of improvisation and their freedom to do so. One mentions how they "rehearsed improvisational lines over games of backgammon or ping-pong." Another includes co-star Isla Fisher in the improvisational mix. Still another provides this quote providing more detail about how they prepared (or didn't prepare) for deviating from the script in scenes:

“The writers wrote a great script and the concept was great, but me and Owen did customize a lot of the lines and do stuff. A lot of it we had done, or we would do prior to shooting. It wasn’t like we would make up the lines as we shot. We would sit and write, come up with stuff prior to shooting the scenes.”

Additionally, Isla Fisher mentions that Vince Vaughn encouraged her to improvise and was very supportive of her efforts when sharing a scene together here.

An expansive document including a film synopsis, actor bios and interviews with members of the production team at Hollywood Jesus (a .pdf-- questionable source, but interesting info contained within) speaks further to the improvisational environment during filming. According to Owen Wilson,

"[y]ou're never going to get the last word with Vince...He improvises a lot and is super fast and glib...Vince had some funny ideas for my character and gave me some great lines to say and sometimes I would have an idea for his character in the scene."

Producer Andrew Panay:

“The real fun began after take three or four when David would let Owen and Vince have a free take to do whatever they wanted. Almost every time they would come up with these gems and it was difficult not to laugh during the middle of the take.”

Director of photography Julio Macat adds a note from his perspective:

“You have to be loose with Owen and Vince so you’re not restricting them to hard marks for the lighting,” explains Macat. “If you’re not loose, you can miss a moment. In certain situations we did things like cover two people from across each other at the same time, which isn’t easy because it ends up being a jungle of flags and cutters. It takes more time to set up, but really pays off with guys like Owen and Vince, who may improvise their lines, but always get to the thematic point of the scene just by saying one little thing that touches a chord.” Dobkin adds, “I think magic happens all the time, especially when you're improvising..."

Director David Dobkin is "...a strong believer in script develeopment," continuing "Owen and Vince always make significant and substantial contributions to story and character."

Finally, after comparing a handful of quotes mentioned in the IMDb 'Memorable Quotes' page for the movie to the script located here, many popular lines are not present in the script (though naturally, there are a few exceptions).

Some Examples:
"Holy Shirts and Pants;"
"I'm reading don't-kill-myself books;"
" I'm not perfect. But who are we kidding? Neither are you. And you wanna know what? I dig it!;"
" Gloria, I apologize to you as I don't seem real eager to jump into a forced, awkward intimate situation...;"
Jeremy's 'confession' to the priest;
"That's not how you cut cake, you gotta treat cake like a lady!;"
"people helping people;"
"Grab that net and catch that beautiful butterfly, pal!"

Some Exceptions:
"We're all one;"
"massage oils and a book on Tantra;"
"I can do it myself __!"

I concede that discrepancies between this script and the actual film do not necessarily indicate heavy improvisation, but they may -- and that possibility is worth considering in this case, given the quotes above.

So to answer your questions @SethRogers: yes, they improvised and yes they deviated from the script-- heavily at times.