Bold the name of the speaker on specific frame

I have three speakers on the title frame as follow:

\author{Mister X, Mister Y, Mister Z}


I want to bold the name Mister X on frame 1 and 2 and Mister Y on frame 3 and 4 for example. I haven't found the solution; the content of \author can't be overwritten.

It's possible to do that with beamer?

Regards

Here is a small code to illustrate:

\documentclass{beamer}
\usepackage[utf8]{inputenc}
\usetheme{Warsaw}
\usecolortheme{crane}
\usepackage[french]{babel}

\title{My Talk}
\subtitle{All authors must be here}
\author{Mister X, Mister Y, Mister Z}

\begin{document}

\begin{frame}
\titlepage
\end{frame}

\section{Mister X's part}

\frame{
\frametitle{Part of Mister X}

"Mister X" must be bold in the footer.

}

\section{Mister Y's part}

\frame{
\frametitle{Part of MisterY}

"Mister Y" must be bold in the footer.

}

\end{document}

-
can you include a small example together with the theme you are using for us to test? –  percusse Apr 6 at 14:44
Welcome to TeX.sx! Please add a minimal working example (MWE) that illustrates your problem. It will be much easier for us to reproduce your situation and find out what the issue is when we see compilable code, starting with \documentclass{...} and ending with \end{document}. –  Marco Daniel Apr 6 at 14:45
I've edit my message to add example code :) –  J0llyJumper Apr 6 at 14:56

You can test the number of the frame by \value{framenumber} and modify the optional argument of \author. The conditional \ifcase test the number of the value and use the given definition. Note that \ifcase starts at 0.

\author[%
\or%1
Mister X, Mister Y, Mister Z
\or%2
\textbf{Mister X}, Mister Y, Mister Z
\or%3
Mister X, \textbf{Mister Y}, Mister Z
\else%4,5,...
Mister X, Mister Y, Mister Z
\fi%
]{Mister X, Mister Y, Mister Z}


\documentclass{beamer}
\usepackage[utf8]{inputenc}
\usetheme{Warsaw}
\usecolortheme{crane}
\usepackage[french]{babel}

\title{My Talk}
\subtitle{All authors must be here}
\author[%
\or%1
Mister X, Mister Y, Mister Z
\or%2
\textbf{Mister X}, Mister Y, Mister Z
\or%3
Mister X, \textbf{Mister Y}, Mister Z
\else%4,5,...
Mister X, Mister Y, Mister Z
\fi%
]{Mister X, Mister Y, Mister Z}

\begin{document}

\begin{frame}
\titlepage
\end{frame}

\section{Mister X's part}

\frame{
\frametitle{Part of Mister X}

"Mister X" must be bold in the footer.

}

\section{Mister Y's part}

\frame{
\frametitle{Part of MisterY}
"Mister Y" must be bold in the footer.
}

\end{document}

-
The solution can be done more elegant if you want to use something like \misterxslide{1,2,3}\misteryslide{5,6,7}. –  Marco Daniel Apr 6 at 15:10