Atra du evarinya ono varda.
If it was a small and friendly-looking legendary like Mew, Celebi or Jirachi, I'll stop and peep at them from behind a bush or something. I'll try not to make any sudden or drastic movements that might scare them off or provoke them to use their attacks against me. I'll also try to stealthily follow them around. If I had a camera with me, I'll definitely snap a few pictures for safekeeping and memories.
On the other hand, if it was a huge, dangerous-looking legendary that looks like it's about to go on a rampage or let loose a blast of Hyper Beam at anything that moves, I'll take one quick picture and get the heck outta there!! D: