If their is a delay for a completion of the Project in a time a Head of the team member should Act as a Boss or a Leader?

Act as leader from front  to complete work within time from my side and encourage people to achieve goal to get to line in time 

When everyone should work together with unit not like superiors or seniority feelings, then only target will achieve in the time. It is my real time experience.

LEADERSHIP IS THE BEST TO GO WITH 

I prefer to be a leader because in failure leader should take a responsibility and  he should lead from front and also inform team members to support and give them the motivation and advice, I am sure after doing this team will be able to finish the project within the stipulated time and Once project is finished a good leader should give the complete credit to its team members and it will give very good boost to them.

Undoubtedly, he should be a leader !

First thing is he should take the responsibility for the delay & then educate the team for the present scenario. Don't point out if a particular person is the reason for that, instead tell him what has happened and make sure that he works better than before.

i think he should act like a leader while the time of project is not over .. after that  he should act like a boss whether the mission was completed in time or not

I will prefer to be a leader in such situation . Because the team members will be more close to the leader than their boss. A leader can understand exactly what are the requirements and the exact problems may occur while completing the tasks. Leader can act as player as well as a controller by giving proper instructions and promoting team spirit . 

I think like a leader while the time of project is not over.

i think we should work as team and work together. leaders always cooperate and work with their members.