He starred in the successful Slovak film Music, where he played the main character -- musician Martin Junec, who loves jazz bit is not quie so sure of his marriage. For his portrayal, Kostelny received the Slovak prize Slnko v sieti (Sun in Net), so-called Slovak Oscar from the Slovak Film and Television Academy.

He further enhanced his reputation by acting in Vladimir Michalek's Czech film About Parents and Children, playing the son of the main character.

His face became known to TV viewers thanks to TV series launched by Slovak commercial television stations Markiza and Joj. In Ambulance in the Rose Garden for Markiza TV, he plays a young gynecologist. In the new series Professionals at Joj TV he was cast as a policeman.

Kostelny's previous film credits include Martin Sulik's The City of the Sun, or Working Class Heroes, and in Zuzana Liova"s film Silence. He studied at the Academy of Performing Arts in Bratislava and is a member of the Slovak National Theatre company.