When EastEnders was first introduced, its creators tried to avoid casting actors who the viewing public would recognise as "stars". The two main aversions to this were Wendy Richard and Susan Tully.
Indeed, many alumni of Series/Grange Hill ended up appearing in EastEnders. As well as the afore-mentioned Susan Tully (Suzanne Ross in Grange Hill and Michelle Fowler in EastEnders), both actors to play the role of Mark Fowler appeared in Grange Hill.
Former EastEnders actors had a habit of turning up as characters in The Bill: Irene Raymond (Roberta Taylor) = Gina Gold, Mark Fowler Mk II (Todd Carty) = Gabriel Kent, Ruby Allen (Louisa Lytton) = Beth Green, Natalie Evans (Lucy Speed) = Stevie Moss etc.
And vice-versa, especilly after the plug was pulled on The Bill.