Starting text right after section or subsection title

Question

Is it possible to start your text right after the section or subsection title (without starting a new line)? Thank you. I hope the question is clear enough and does not need a MWE.

Answer 1

In the default classes the run-in style is used for \paragraph and \subparagraph to change \section to use the same find it's defintion, in article it is

\newcommand\section{\@startsection {section}{1}{\z@}%
                                   {-3.5ex \@plus -1ex \@minus -.2ex}%
                                   {2.3ex \@plus.2ex}%
                                   {\normalfont\Large\bfseries}}

and you need to invert the sign of the 3/4th arguments as that is used as a switch to specify run-in style so

\renewcommand\section{\@startsection {section}{1}{\z@}%
                                   {3.5ex \@plus 1ex \@minus .2ex}%
                                   {-1em}%
                                   {\normalfont\Large\bfseries}}

Also changed the 4th arg to specify 1em space after (rather than below) the heading, as for \paragraph

Answer 2

David has already provided the package-free solution; here's another option, using the titlesec package (simply selecting the runin predefined format):

\documentclass{article}
\usepackage{titlesec}

\titleformat{\section}[runin]
  {\normalfont\Large\bfseries}{\thesection}{1em}{}
\titleformat{\subsection}[runin]
  {\normalfont\large\bfseries}{\thesubsection}{1em}{}

\begin{document}

\section{Test Section} 
Test
\subsection{Test Subsection} 
Test

\end{document}