OK, so normally, friends wouldn't address each other formally, in Czech, even though the vocative case is used? Am I understanding this properly? So if Matěj is the speaker's friend, the sentence would be, "Matěji, ty jsi dobrý přitel," correct?
BTW, this is so similar to Ukrainian!
Matvíju, vy/ty [je] dobryj pryjatel'.
Матвію, ви/ти [є] добрий приятель.
Thank you both VladaFu and va-diim. That does help, to know that the consonant sounds different because it is unvoiced. I looked on Youtube and I was surprised and pleased to see that there are several videos entirely dedicated to pronouncing that sound! I can at least roll my rs, so I will practise.
Přítel means often "boyfriend" if a lady or woman says that about a man.
Kamarád means now just a friend, used to be (and maybe it still to some extent is) more colloquial than přítel, and used to be used especially in certain contexts say 100 years ago, but is now often used instead of přítel when a confusion with the "boyfriend" meaning is to be avoided.