(This refers to the secondary characters and love interests who lie to the MC, not the MCs with dishonest streaks, since the latter can be a great deal of fun.)
Anyhow, when writing a first person POV, facts presented by other characters typically have to be taken at face value. Unless the MC/reader has already seen/heard something that contradicts what the other character is saying, or unless that character is a notorious liar, the things they say are pretty much accepted as truths.
On the one hand, this can be used to create some shocking plot twists. On the other hand, if you knock the reader's feet out from under them too many times, they're going to get annoyed. Even if you only do it once, if a shocking plot twist had no foreshadowing - a couple of events/bits of dialogue the reader can look back on and think, "Oh, now I get it!" - I think we run the risk of jarring readers out of the story and perhaps into closing the book.
And yet, if the foreshadowing spells out that Character B was lying to Character A, to the point where the reader is banging their head against the wall, shouting, "Why haven't you clued in, Character A!?!?" that's not good either.
Surely the possibility of a happy medium exists, but I find it can be very hard to achieve. We, as the authors, know everything. Sometimes it's hard to gauge what a clue or bit of foreshadowing will tell the reader. Is it enough? Is it too much? And then there's always the problem: would this actually happen?
Would Character B actually slip up and say something to make Character A suspicious? Would they leave the mysterious note lying around where Character A could find it? Would they be creeping around, going about their mysterious business outside Character A's window late at night?
In plenty of cases, yes, they might.
But personally, I've found myself having to say no a lot of the time. This is because, in my WIP, I'm dealing with not one but two very good liars. I've got one ex-Intel agent who was trained in the art of deception, and one military captain who's been lying to people like my ex-Intel agent almost constantly for the past seven years. Furthermore, both their lives are on the line, so they're going to be darn careful about what they say and do.
It's had me really quite stuck for a while, and it doesn't help that my MC isn't the brightest crayon in the box, so she doesn't question things too much. Which might drive my future betas insane - I really can't tell -- but anyway!
There was a point to all that ranting and babbling. Honestly, there was!
I want to know what you think of all this. What sort of hints do you give your readers about the things other characters are hiding? What sort of hints do you want as a reader? How clear/obscure do you like things to be, how long can you tolerate mysteriousness dragging on, and/or how badly annoyed are you when an author rips a plot line out from beneath you and turns it around in the opposite direction?
Let me know! I'm curious. :)