"[M]ost of the time" is not necessarily so. Some actors will play the off-camera lines if they can. In fact, some actors consider it rude if the other actors won't do that. A director I know who knows Tom Hardy said Hardy told him that Stewart wouldn't play his off camera lines to him and that it hurt Hardy's performance.
I could be wrong (I don't have the data in front of me) but my recollection is that they did Montalban's scenes early in the shoot, meaning they pulled the TMP bridge apart to rearrange it for the Reliant, then put it back together as the Enterprise bridge and moved some of the stations around as compared to TMP. That the Enterprise bridge was used in pickups near the end of the production tends to support this.