Some related topics:
Boundaries are very specific responses to very specific difficulties. They can come in the following forms: (1) I say no ('I can't' / 'I don't want to') and (2) (to someone else) please stop / start doing something / please do X differently. They're moves in a negotiation of how the common activities are devised and regulated.
Openness, by contrast, is fluid, and what I disclose to whom and when, and what I join in with and do not join with, changes from day to day and situation to situation and person to person in an unpredictable way. This must be governed by instinct, not fixed rules. This requires a great degree of alertness, acute observation, and agility.
Candour is disclosure; honesty is straight-talking and straight-dealing.These overlap but are not coterminous. True honesty requires discretion. Candour is reckless.
Intimacy does not require absolute openness and full disclosure of all details. Being private does not mean evasion and does not constitute a blockage to intimacy. Being candid does not necessarily mean being direct and facilitating intimacy. It is possible to feel extremely close with someone who verbalises only little of what is going on inside them, and someone who verbalises everything can be very alienating and disconcerting. Openness and intimacy are quite different topics and should not be conflated or correlated.
A related question is trust.
An unhelpful notion of trust is proceeding on a contractual basis with another person, governed by a roster of behaviours that must be exhibited perfectly by the other person (usually total candour, incorrectly termed honesty, and total fidelity in thought, word, and deed), accompanied by monitoring, and vicious chiding when (not if) the other person fails. The person demanding that they can 'trust' the other person is essentially looking for a perfection neither can deliver, and will rove from relationship to relationship, Princess looking for their Price. This person cannot trust. They temporarily take possession of others, claim to trust, but really sleep with one eye open. The real goal of such a person is the victimhood and moralisation they can indulge in when (not if) the other person fails.
Real trust is this: sticking with someone even though they have character defects and off days, weeks, months, or years, because you are confident that things will work out anyway, and you love them beyond their virtues and defects.