It takes from the dramatic effect of his choice to have Portia clap her hands to her face in pleasant anticipation just as Bassanio turns and looks at her when he approaches the lead casket. This reaction would tell him instantly which one to choose This undermines the impartiality she's supposed to exhibit as the suitors choose their fate, and thus the wisdom of Bassanio's..
to have brett play basanio they need a different portia and to have ploughwright play portia they need a differnt basanio. brett is too good looking for the very plain ploughwright. it is bad that portia's assistant is more handsome than portia.
