Should You Learn the Other Guy's Lines?

I am busying learning a new batch of lines for my upcoming role in my local theater's Christmas play. I've got most of them down -- but I've still got a long way to go before I feel comfortable on stage.

Some actors have their own special way of memorizing lines. Some constantly listening to their own recording of the dialogue. Others get their willing friends and family members to run lines with them. My system is pretty simple: repetition, repetition, repetition.

But during rehearsals, I've noticed something. I know my lines pretty well. But I have no idea what my fellow actor is going to say. Sure, I know what my cue is -- but I haven't spent much time learning the rest of their dialogue.

Some actors want their reactions to stay fresh, so they don't want to memorize the lines of their co-stars. And of course, one can't help but learn most of the other actor's lines simply by osmosis. Yet some thespians purposefully learn every word of the play, so that if anything goes wrong they can save the day and keep the show on track.

What do you think? Should you learn the lines of your fellow actors?