Since Nora Allen's murder is the major driving force behind first the trip that brought him to Starling City on Arrow and later his superheroic career, it's fairly safe to assume that Harrison's part will be either small or that the pilot will rely heavily on flashback (no pun intended). The part is identified as being a recurring/guest starring role.
The show will feature a modern interpretation of Barry Allen--one whose mother was murdered and whose father was wrongly imprisoned for it. During his guest spot on Arrow, series star Grant Gustin described the strange and harrowing experience of seeing his mother killed by a seemingly-supernatural blur.
In the comics, she was murdered by a time-traveling villain who shared Allen's super speed powers.