Your dialogue is stiff because everyone is being too honest
Great dialogue can come from radical emotional honesty. But in most real conversations, people
dodge questions
answer indirectly
lie (badly)
change the subject
crack jokes to avoid serious answers
lie (convincingly)
say half of what they mean
tell the truth but get so anxious/angry/defensive others now think they're lying
If your dialogue feels unnatural, it might be because everyone is saying exactly what they think, or articulating their thoughts too well. Try letting characters hide things instead.
Subtext is where tension lives.
Also a lot of people can just be bad at communicating things between eachother or have conditions altering their communication / listening skills.
examples:
did'nt think that particular bit of information was important enough to bring up
mistook the tone of the conversation
wildly differing opinions on the situation between characters
forgot to inform that guy on something important because the though got overshadowed by more pressing issues in the moment and everyone assumed somebody had told them, when they infact did not
character is a bit stupid and acted like they understood some of those big words even though they considered it meaningless gibberish supposed smart people use to sound smarter than they are
character is distracted by some other thoughts or things durring the conversation
they are actively disociating due to recent events
doesn't feel comfortable pressing for answers from a friend who isn't being completely open about what they know
is so madly infatuated with another character that their brain makes everything that character says sound better than it is
not the most coherent thinker in general and it's hard to know what they know or don't at any given moment
choosing wording very carefully as not to anger another character who happens to be in the room at the moment
is blissfully unaware that what they have just said is actually deeply insulting in this situation
doesn't know how to give constructive criticism or compliments properly due to having a bad upbringing
zero sense of personal space that extends into asking invasive and personal questions that often leave others feeling uncomfortable
feels like they will die if people find out who they really are as a person and not the fake personality they use for show
Stiff dialoge can also potentially be a side effect of not knowing your characters well enough or not thinking enough on how things might have affected them. Nobody thinks entirely the same way, and the same goes for speach patterns and conversational habits.
Do they gesture with their hands a lot while talking? Do they site sources sometimes to back up what they just said? When was the last time they told somebody that they loved them? And how would these kinds of things alter the flow of conversation given the context?
I'm not saying these should be used one-to-one with the given examples. This is an exercise to help break out of restrictive patterns in dialogue.
In real life not everyone is completely honest or in the right state of mind, So to feel dynamic you either need good enough reason to justify in context why your characters speak and act the way they do if it comes across as odd, or make their personal traits leak into how they interact with the world without having to say it directly.
