Born in Vlădești, Argeș County, he graduated high school in Câmpulung in 1967 and Bucharest's Caragiale Academy of Theatrical Arts and Cinematography in 1971. He made his theatrical debut a year before graduating, at the Bulandra Theater, in Truman Capote's The Grass Harp. His screen debut came in Dan Pița's Nunta de piatră (1972); he would later appear in the same director's Filip cel Bun (1975). Other directors with whom he has worked include Alexandru Tatos (Mere roșii, 1975; Secvențe, 1982; and Secretul armei secrete, 1988), Virgil Calotescu (Buletin de București, 1982; and Căsătorie cu repetiție, 1985), Nae Caranfil (Asfalt Tango, 1996; and Filantropica, 2002) and Cătălin Mitulescu (The Way I Spent the End of the World, 2006). In all, he has appeared in more than sixty films. Diaconu worked with Lucian Pintilie both in film (the 1981 De ce trag clopotele, Mitică?) and in theater: Gogol's The Government Inspector at Bulandra, and Shakespeare's As You Like It at the Nottara Theater. Liviu Ciulei hired him at Bulandra in 1972, and he remained there for a decade, before moving on to Nottara. At Bulandra, he appeared in Twelfth Night and The Tempest, both under Ciulei's direction; and in Marin Sorescu's Răceala, directed by Dan Micu. At Nottara, he appeared in Alexander Ostrovsky's The Forest; Ultimul bal, after Liviu Rebreanu's Forest of the Hanged; and Mihai Ispirescu's Într-o dimineață. He became the director of Nottara Theater in 2004, working as such until his resignation in 2011.

Filmography

• 2015To Paris with the Identity Card — Drăgan Niță
• 2014A Love Story, Lindenfeld
• 2014In-laws — Ştefan Vrânceanu
• 2010Poker — Calabrezu
• 2009Kino Caravan — Tanasie
• 2008One-Day Adventures
• 2007Tache — Tache
• 2007Ticăloșii — Mircea Prejbeanu
• 2006Happy End
• 2006Love Sick — Mr. Dragnea
• 2006The Way I Spent the End of the World — Grigore Matei
• 2003Toma Caragiu - Momente de aur
• 2002Philanthropy — Ovidiu Gorea
• 2000The Firemen's Choir
• 1999Fii cu ochii pe fericire — Titi
• 1996Asphalt Tango — Andrei
• 1996Dănilă Prepeleac — Dănilă Prepeleac
• 1992The Telephone — Radu
• 1990Reach for the Sky — Mircea
• 1989Happenings with Alexandra
• 1988Flames Over Treasures — Barkeeper
• 1988O vară cu Mara — Ilieș Calancea
• 1988The Secret of the Secret Weapon — Zmeul Zmeilor
• 1987A Smile Of The Sun — Împăratul
• 1987Starting Over — Mircea
• 1985Promisses — Petruș
• 1985Repeated Wedding — Radu Petrescu
• 1985The Kid — Writer
• 1984A Light on the 10th Floor — zidarul Trică
• 1984Horea — Candrea Alexa Kendy
• 1984Un petic de cer — Ion Panait
• 1983Căruța cu mere — Garofiță
• 1982Bucharest Identity Card — Radu Petrescu
• 1982Sequences... — motel keeper
• 1982The Computer Testifies — V. Vișan
• 1982The End of the Night — Prosecutor Matei
• 1982Waiting for a Train — Zarzalin
• 1981The Oil, the Baby and the Transylvanians — Romulus Brad
• 1981The Sign of the Serpent — Flondor
• 1981Why Are the Bells Ringing, Mitica? — Iordache
• 1980Fox Hunting — Pătru cel scurt
• 1980Poor Ioanide — milițian
• 1980The House Between the Fields — Ilarie
• 1979Tragic Vacation — Sandu
• 1978Before the Silence — Pvt. Dumitru
• 1978Operation 'The Bus'
• 1978The Actress, the Dollars and the Transylvanians — Romulus Brad
• 1978The Prophet, the Gold, and the Transylvanians — Romulus Brad
• 1978This Above All
• 1978Together Again — Bănuță
• 1977A Lost Letter — Iordache Branzovenescu
• 1977Oil — Grigore
• 1977Shootings Under the Moonlight — Sgt. Marin Văduva
• 1976A Summer Tale
• 1976Red Apples — Dr. Irod
• 1975Filip the Kind — absolventul de liceu Filip
• 1975The Actor and the Savages — Radu Toma
• 1974The Trap — Oarca
• Un august în flăcări
• 1973The Poseidon Explosion
• 1973The Stone Wedding — dezertorul
• 1972Farewell, Dear Nela! — Bibi

Appearances as themselves (1)

• 1992Videograms of a Revolution — himself (archive footage)