<RichTextBox>
    <FlowDocument>
       <Table>
           <Table.Columns>
              <TableColumn />
           </Table.Columns>
           <TableRowGroup>
               <TableRow>
                   <TableCell GotFocus="SelectionCell_GotFocus">
                       <Paragraph>1</Paragraph>
                   </TableCell>
               </TableRow>
           </TableRowGroup>
       </Table>
    </FlowDocument>
</RichTextBox>

<RichTextBox IsDocumentEnabled="True">
    <FlowDocument>
        <Table>
            <Table.Columns>
                <TableColumn />
            </Table.Columns>
            <TableRowGroup>
                <TableRow>
                    <TableCell GotFocus="SelectionCell_GotFocus">
                        <BlockUIContainer>
                            <Canvas>
                                <Button Click="Button_Click">
                                    Click
                                </Button>
                            </Canvas>
                        </BlockUIContainer>
                    </TableCell>
                </TableRow>
            </TableRowGroup>
        </Table>
    </FlowDocument>
</RichTextBox>

void MyCode()
{
    Paragraph p = new Paragraph();
    p.MouseEnter += p_MouseEnter;
}

void p_MouseEnter(object sender, EventArgs e)
{
    Paragraph p = (Paragraph)sender;
    p.Background = Brushes.Red;
}

**In Markup**

<Paragraph MouseEnter="p_MouseEnter" />

**You can also trigger on properties like IsMouseOver in styles**