Orochimaru is making Sasuke do his dirty work again. At least this time it only involves painstakingly drawing out a seal large enough to fit a person on a smoothed surface prepared specifically for this, rather than any wet-work. Sealing is dangerous if he puts a single stroke wrong, of course, but it's unlikely to lead to him being disciplined by his 'teacher.' Intentionally sabotaging it to spare Orochimaru's next victim wouldn't work here, after all.
He finishes the last stroke on the inner circle, sharingan active to make sure it's precise enough.