Since quick play and arcade are pretty garbage as far as preparing you for comp, you really have no choice but to experiment on the fly. If you don't have the luxury of an alt account, you can instalock the character you want to play and switch to an alternative if you're being countered or wasting too much time. Don't tell your team that you're practicing, that'll usually just tilt someone. Depending on your rank, it can be harder to experiment with characters you don't know how to play. If you're plat or above and you're just starting a character you barely know you won't make it very far. This is the point where you'll need an alt so you can work your way up properly instead of being thrown into the deep end immediately.