Well...the dalek Rose was dealing with was essentially dying, vulnerable and not a threat...the one River encountered was getting stronger(and if I remember correctly had just zapped the Doctor) Let's have a little context here. Geek, Beats, The Doctors, Rose Tyler, Rivers T-Shirt, Doctors Who, Rivers Songs, Dr. Who, Donna Noble
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they both had to laugh, because they know that unknown, the monsters, the running is the best part. No chance is either of them going to give that up. No way they're just going to ignore whatever is going on here.