It highly depends on the director. Some directors (mainly the ones who were actors themselves once upon a time) allow the actors to play out a scene a little here and there. These types of directors will encourage it. Don’t forget they often take many takes of a scene. Some actors are devilishly brilliant when it comes to saying or exploring their lines in a different way. Imelda Staunton, in every take, says her lines differently. She doesn’t add words mind, but the emphasis is on different parts of the sentence. This is slightly moving away from your question, however.
The reason you had been so surprised is because Writers are friends, what does an actor know about how something should be said? Writers, assistant writers, script assistants etc have worked so hard on making a scene as perfect as possible. Now, we all know how amazing Breaking Bad is, but a lot of blood and sweat went into making that script so excellent. The actors came to the process much, much later. During this period of script preparation so many lines had been cut, added, rearranged, torn up, thrown at the highest trash can in the room. It’s a process of extreme heartache, unless you are Shakespeare, of course. Even then I expect he had a small moan once in a while :-)
An actor who goes into a shoot, remember, there’s very little rehearsal for TV and Film and says this and says that when it’s not in the script will likely never be hired again. An actor has to serve that text, in the Theatre it’s called DLP, Dead Line Perfect. Every word, said as it is written, perfectly.