Workflow: Can we set a task form default value from a workflow variable?

yes, it's possible

Thanks for the reply. Is it possible to achieve it without Nintex forms. We don't have Nintex forms.

I'm not sure.

what form solution do you use?

I believe the only other option I have is to use InfoPath form

I do not have experience with infopath buat I would say it might be possible How to set default username to PeoplePicker in infopath form? 

Thanks Marian. But this is slightly different. Its easy to get the user name on InfoPath directly. But the challenge come when you want to populate the people and group field on the InfoPath task form, and that too dynamically based on some data from the item that the workflow is running on. 

Really appreciate for researching on this.