It’s common knowledge among fans of CW’s Arrow that there are plans to have the character Flash, played by Grant Gustin, to appear in three episodes this season – eps 8, 9, and 20 – with 20 being a backdoor pilot hopefully intended to launch the character’s own series. Both episodes 8 and 9, which only introduce the character Barry Allen sans costume, have already been shot but the 20 seems to have been put on hold in favor of just shooting a traditional standalone Flash pilot.
It has been said that the change in plans was the result of the CW brass seeing cuts of episode 8 and 9 and feeling confident that the character can carry its own show. One advantage of a stand alone pilot over the backdoor Arrow pilot is that the creators have more freedom when it comes to fleshing out the Flash’s backstory, particularly because pilots usually have higher budgets than normal episodes.
There’s currently no word on what episode 20 of Arrow will be about and whether it will still feature Barry Allen.