The trust you'll build over time will be subtly undermined by a feeling that neither one of you will be able either to pinpoint or ignore. Perhaps your own projected images of an idealized friend will eclipse the person they truly are, and when their real personality shines through, you'll feel confused or disappointed. They could feel the same way about you, especially if you engage in small ways of misleading them.
The key to building a strong foundation of trust is open communication. If you two have plenty of strong communicative links between you, those will help ease the confusion and difficulty of this aspect. If not, you'll need to pay extra close and careful attention to the ways in which you represent yourselves in conversations, arguments and daily life. Be sure you're not sending out mixed signals.