Phil Parkinson's men go into Friday night's second leg with it all to do having lost out to Millwall 3-1 in the first encounter between the two sides at the Coral Windows Stadium last weekend.
Darby has faith in his team-mates to do themselves proud in the capital, but he also knows City will have to put in one almighty shift if they wish to keep their season alive.
The 27 year-old said: "It is important when we go down to their place that we come out of the blocks firing and show the same character we did in the second half last Sunday.
"We have to show everyone how much we want it.
"We have worked hard all season to get into the Play-Offs but we need one more massive effort required by all of the lads.
"We have ninety minutes left in this tie, maybe more, to give it everything we've got and to show people what it means to wear the Bradford City shirt.
"We have to dig deep and really take the game to them.
"It is important that we are on it right from the first whistle because we know how tight it is between the two teams and how key the fine margins are.
"If we get the first goal, then it's game-on.
"We have a really experienced dressing room and we know what is expected of us.
"There is plenty of character within our group and there is no better time to show it than down at Millwall on Friday night."