At a bare minimum, your LaTeX source file needs to contain these elements for \citeauthor to work:
---- doc.tex ----
\documentclass{article}
\usepackage[numbers]{natbib}
\begin{document}
\citeauthor{goossens93}
\bibliographystyle{plainnat}
\bibliography{doc}
\end{document}
---- doc.bib ----
@book{goossens93,
author = "Michel Goossens and Frank Mittlebach and Alexander Samarin",
title = "The LaTeX Companion",
year = "1993",
publisher = "Addison-Wesley",
address = "Reading, Massachusetts"
}
The correct compilation cycle for the document is then as follows:
latex doc
bibtex doc
latex doc
latex doc
The first compilation generates the list of cited works for BibTeX. Then BibTeX uses that information to generate a .bbl file from doc.bib containing all of the citations formatted according to the bibliography style you choose. The second compilation incorporates the .bbl into the document, and the third compilation updates all of the cross-references (e.g. citations).
Here is the output for the example above:

Also for some reason I don't see the indexes (eg. [4]) when using plainnat. Why?
– Alexandru Jun 03 '09 at 20:26instead of
[4] Author, Title,
I get only
Author, Title
I tried \usepackage[numbers]{natbib} and "plainnat" with no success.
– Alexandru Jun 04 '09 at 09:11\bibliographystyle{unsrt}with command\citeauthor{some_id}produces (author?), whereas using\bibliographystyle{unsrtnat}with\citeauthor{some_id}produces the desired result. – tjanez Sep 03 '13 at 08:30