Not in your real life; I’d think that’s a loaded question with too many answers to count. But in writing, things are a tiny bit simpler. Tiny bit.
There are two main different situations where you might find yourself writing a two-faced person.
THROUGH POINT-OF-VIEW CHARACTER
This situation can be as complicated or as easy depending upon how self-aware the character happens to be.
An incredibly self-aware character, one who knows and accepts their two-facedness, can easily show their true colors through their interactions with other characters and through their own internal thoughts. Their internal thoughts will align completely with their actions, giving the reader a double whammy of explanation. This is where you can write simple, uncomplicated statements, such as “He/I lied” or “He/I didn’t care who he/I hurt” inter-spaced with other, longer internal motivation that will bolster the character’s actions and give the reader a complete sense of what kind of character they’re dealing with.
This is, by far, the easiest two-faced character to create. However, if the character is completely morally ambiguous, you’ll have a much more difficult time convincing the reader to have empathy for him. To improve empathy, you’ll have to show his likableness by 1) having him engage in ‘nice’ or ‘kind’ behavior, 2) by showing other people enjoying his presence or comparing him favorably, or 3) by giving him a clearly defined motivation that readers can identify with.
A character lacking in self-awareness (a type of unreliable narrator), will cause slightly more difficulty given their actions and their thoughts will not align. This is the character who thinks of himself as correct, moral, or a victim in situations rather than a perpetrator. A character who does not take responsibility for the negative outcomes of his actions because he believes in his own false narrative. In this situation, you can’t write “He lied” ever because as far as the character is concerned, he isn’t lying.
Sometimes, a reader might mistake this unreliable character as being truthful (think about how, culturally, the name Lolita represents a young sex fiend rather than a victim), and this is generally the better outcome for the author. The worse outcome for the author is to have the readers think that the author is merely incompetent, incapable of aligning the thoughts and words correctly for the character. So this takes finesse.
Some advice here would be to 1) clearly show secondary character reactions. Don’t skimp over those reactions. Make sure that if there is an issue, it is called out. Whether it’s by a designated secondary character who represents a moral compass, or by a group of secondary characters, each of whom is responsible for opening the reader’s eyes just a little bit more.
Another trick would be 2) to insert doubt into the point-of-view’s internal monologue at certain times. By showing a level of doubt or internal argument as he convinces himself that what has been said about him is not correct (despite it actually being correct) you allow the reader to also engage with those same doubts and question whether this character is, in fact, not as reliable as once thought.
THROUGH A SECONDARY CHARACTER
Within a secondary character you lose two things: 1) internal monologue and 2) multiple character interactions.
Losing internal monologue will make it so your reader can no longer match up actions with thoughts because those thoughts are MIA.
[This can cause problems in romantic tension when you only have one point of view between the two love interests. Without that second person/love interest capable of explaining their actions away through their own internal monologue, the reader can easily decide that the character is simply awful after a particular negative sequence. This is one of the main reasons you see romance flip back and forth between two characters’ heads, so the reader will never be left in doubt as to how much the two people care or want each other.]
When writing two-faced characters, it’s not imperative to show the secondary person’s internal monologue, if 1) the author can show it plainly as subtext within the character’s actions and 2) the point-of-view character acknowledges the inconsistencies at some point to affirm reader suspicions.
As for losing multiple character interactions, this is a problem that can be jumped over by tricky means. These means include, but are not limited to: having the main character over hear/view the two-faced one’s separate interactions via video-tape/magic ball/eavesdropping, etc. (think Julia Roberts watching that video in Ocean’s Eleven), have the main character read diary entries/work contracts/emails, etc., have the main character be presented with differing accounts by many different secondary characters (this is what is used in the vast majority of detective murder mysteries), have the two-faced character reveal their own two-facedness themselves on purpose (usually a scene where a friend or colleague turns their back).
Losing these two elements of a character can either provide a higher level of benefit, as in the case of a mystery, or a higher level of difficulty with less return, as in the case of romance. If combining the two genres, it’s important to go the extra mile to make sure that the secondary character, even in the face of sneaky or underhanded behavior, is given the chance to air his true thoughts in some fashion.
As a personal example: The reason I never go into Ari’s point-of-view in Bridle the Unicorn is because he knows too much that he doesn’t want to say, which is essentially giving him a level of two-faced behavior.
Aaaand this got way longer than I thought it would.