Someone is running down the side of a mountain.
They are sitting at the symphony.
'Someone' implies one person and if he is running down a mountain, he is outdoors and on his feet whereas 'they' implies more than one person and when they are sitting at a symphony, they are sedentary and likely indoors.
exists e1 s1 x1 x2 x3.(n_side(x2) & a_down(s1) & Manner(e1,s1) & Theme(e1,x2) & Actor(e1,x1) & v_run(e1) & of(x2,x3) & n_mountain(x3) & n_person(x1))
exists e1 x1 x2.(n_symphony(x2) & n_thing(x1) & at(e1,x2) & Actor(e1,x1) & v_sit(e1))