A group of people mill around while a man in a fish costume descends a staircase.
a man in a costume comes down the stairs for a party
There could be different reasons the man is in the costume.  We cannot be sure that it is for a party.
exists e1 p1 s1 x1 x2.(exists e2 x3 x4 x5 x6.(Theme(e2,x6) & Actor(e2,x3) & v_descend(e2) & n_staircase(x6) & in(x3,x4) & n_costume(x4) & of(x4,x5) & n_fish(x5) & n_man(x3)) & while(e1,p1) & a_around(s1) & Manner(e1,s1) & Actor(e1,x1) & v_mill(e1) & of(x1,x2) & n_people(x2) & n_group(x1))
exists e1 x1 x2 x3 x4.(n_stair(x3) & down(e1,x3) & for(x3,x4) & n_party(x4) & Actor(e1,x1) & v_come(e1) & in(x1,x2) & n_costume(x2) & n_man(x1))