Believe me, this is not a rule to forget. When I said "Dead Man," I mean dead as in murdered and six feet under. Not good.
Unfortunately, I've broken this rule. Several times. And did I mention that withholding information could count as lying, depending on Scully's mood?
EXAMPLE: Nowhere, Texas. (Well, ok, Northern Texas, but it felt like nowhere.) Anyway, Scully and I went chasing after tanker trucks. I neglected to mention to her that those tanker trucks might have been holding an extraterrestrial virus.
She was pissed when she found out. Very pissed. She began to explain to me that it was "fitting" (note the sarcasm) for her to be with me, chasing phantom tanker trucks, when she had to attend an important hearing in eleven hours. And did I mention we were at an apparent dead end?
Scully really knows how to pile on the guilt. She hadn't wanted to come in the first place, but I'd convinced her (and she'd seen something interesting that she wanted to talk to me about, but that could've waited). Now we were at a dead end, or at least it looked like one. I was ready to call it quits so she would at least talk to me by the end of the week. The only reason I didn't get any more lecture-time that night was because we did find the tankers. They went right by us.
EXAMPLE: Cell phone pranks. I know, I know, doesn't sound like anything major. Just a harmless little white lie, letting her believe you're somewhere else.
Scully doesn't think it's funny. It's the famous Scully Glare when she turns around to see you behind her. That is, after the shock.
Of course, she knows you're trying to be funny. So she tolerates it more than other things, like, say, hiding evidence. So she might get you back.
EXAMPLE: Hiding evidence. Big no-no. To her, it means you don't trust her. Which makes her think you might be protecting her. And that leads to the consequences discussed in Ch 1.
EXAMPLE: Informants. Better tell her if you get a new one. She doesn't trust them, and wants to know about them. That way, if you're led on a wild goose chase, she can blame you for trusting them.
It is not a good idea to lie to Scully. Not at all. Very hazardous to your health. I've only given a few examples, because I really don't like to admit how many times I've gotten in trouble over this. Let's just say a lot. Probably twice a year, at least, but likely more than that.
Lying to Scully. Just don't do it.