The phrase "act in the capacity" is correct and usable in written English.
It is typically used to describe someone performing a role or function in a specific position or authority.
Example: "She was asked to act in the capacity of project manager during the team's absence."
Alternatives: "serve in the role" or "function as".
You can file lawsuits but our tax dollars also go to fight those lawsuits, too bad we can't get a law passed that takes the money for government lawyers out of their bonuses of government employees if found that they didn't act in the capacity to which they were hired…in other words, to protect the land for future generations.
If any dealers are to act in the capacity of sub-underwriters and are to be allowed or paid any additional discounts or commissions for acting in such capacity, a general statement to that effect will suffice, without giving the additional amounts so paid or to be so paid.
Mr. Reider, who does not act in the capacity of a psychologist when he is attending meetings, described the first year after the breakup of a marriage as a "period of insanity". Many men, he said, do not know where to turn for help.
Thinking longer term, Amazon could act in the capacity of a PBM (either through acquisition or on its own) and establish direct relationships with branded manufacturers.
Carroll began to act in the capacity authorized by the proprietor's commission, and Hart turned to the upper house of the colonial legislature for relief.
When the police react to these encounters on a personal level, they are no longer able to act in the capacity of a professional.
Your wife merely acts in the capacity of foreman".
Much of the success of the evening was due to the excellent management of Mr. Underhill who acted in the capacity of both pianist and conductor.
In addition to his mastery of the equine arts, the ringmaster also acted in the capacity of performance director.
From 1277, Stratton acted in the capacity as Countess Isabella's steward, though he never officially used that title.
She largely blamed the attacks on Gregg Housh, an early Anonymous member who often acted in the capacity of a spokesperson and now describes himself as an "observer" of the group.
