Que vs. quien as a relative pronoun

Question

Are “que” and “quien” mostly interchangeable as relative pronouns when talking about a person? Or is there a subtlety that I’m missing? I always feel a little unsure when I use “quien” because it seems like Spanish speakers use “que” more often.

Answer

Hola A,
yes, que and quien as relative pronouns are interchangeable. The use of quien is less usual, as you say, and sounds a bit more formal than que. 
You need to bear in mind though, that when these relative pronouns are used with prepositions, sometimes we cannot use que on its own, but with the articles el/la/los/las. For example:

Ella es mi mejor amiga, en quien/en la que confío plenamente.
She is my best friend, who I fully trust.
Mi marido es el hombre por quien/por el que lo daría todo.
My husband is the man for whom I'd give everything.
Los chicos a quienes /que/ a los que vi en el parque eran muy guapos.
The guys I saw in the park were very handsome.

Un saludo, 
Inma