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.
2015
as Drăgan Niță
2014
as Ştefan Vrânceanu
2014
2010
as Calabrezu
2009
as Tanasie
2008
2007
as Mircea Prejbeanu
2007
as Tache
2006
2006
as Grigore Matei
2006
as Mr. Dragnea
2003
2002
as Ovidiu Gorea
2000
1999
as Titi
1996
as Andrei
1996
as Dănilă Prepeleac
1992
as Radu
1990
as Mircea
1989
1988
as Zmeul Zmeilor
1988
as Ilieș Calancea
1988
as Barkeeper
1987
as Împăratul
1987
as Mircea
1985
as Radu Petrescu
1985
as Petruș
1984
as Candrea Alexa Kendy
1984
as zidarul Trică
1984
as Ion Panait
1983
as Garofiță
1982
as motel keeper
1982
as Radu Petrescu
1982
as Zarzalin
1982
as V. Vișan
1982
as Prosecutor Matei
1981
as Iordache
1981
as Romulus Brad
1981
as Flondor
1980
as Pătru cel scurt
1980
as Ilarie
1980
as milițian
1979
as Sandu
1978
as Romulus Brad
1978
as Bănuță
1978
as Pvt. Dumitru
1978
as Romulus Brad
1978
1977
as Sgt. Marin Văduva
1977
as Iordache Branzovenescu
1977
as Grigore
1976
1976
as Dr. Irod
1975
as absolventul de liceu Filip
1975
as Radu Toma
1974
as Oarca
1973
1973
as dezertorul
1972
as Bibi