How to remove bold fonts from an entire document?

Question

Following your suggestions below, I would like to modify the colo and the sizers of the chapters, sections, subsections and so on following the style of classicthesis, but with a scrbook class : without any bold font. Is my preamble correct? This is my MWE:

    \documentclass[fontsize=12pt,%
               twoside=semi,%
               headings=small,%
               chapterprefix=true,%
               listof=flat]%
{scrbook}
\addtokomafont{pageheadfoot}{\scshape\small}%changed to small caps in headers
\addtokomafont{disposition}{\normalfont}
\addtokomafont{part}{\scshape\small}
\addtokomafont{chapter}{\scshape\Large}
\addtokomafont{section}{\scshape\small}
\addtokomafont{subsection}{\itshape}
\addtokomafont{subsubsection}{\itshape}
\addtokomafont{paragraph}{\scshape\small}
\addtokomafont{subparagraph}{\scshape\small}

\begin{document}
    \chapter(A title)
    \section{A section}
    \subsection{A subsection in italics}
\end{document}

Answer 1

The KOMA-Script way to change font attributes is using \addtokomafont or \setkomafont. In your preamble, add

\addtokomafont{disposition}{\normalfont}

and you remove all bold fonts in table of contents and sectioning commands (part, chapter, section etc.), without touching the other part of the definition.

See page 54 ff. in the English manual (table 3.2) for a list of elements you can change using these two commands.

Your MWE:

\documentclass[fontsize=12pt,%
               twoside=semi,%
               headings=small,%
               chapterprefix=true,%
               listof=flat]%
{scrbook}
\addtokomafont{disposition}{\normalfont}

\begin{document}
    \chapter(A title)
    \section{A section}
    \subsection{A subsection in italics}
\end{document}

By using the documented interface when changing font attributes, you avoid later possible ‘surprises’. (DISCLAIMER: I am not implying that TeXnician and egreg’s solutions will give you future surprises.)

Answer 2

If you really want no bold font in your document then do \let\bfseries\mdseries in your preamble.

Alternatively you could use \renewcommand{\bfdefault}{m} as egreg suggested.