<TextBlock Style="{StaticResource WordStyle}" x:Name="Foo"><Bold>d</Bold>o o<Bold>dd</Bold></TextBlock>

Foo.Inlines.Clear();

// Bold text.
var block = new Run();
block.Text = "d";
block.FontWeight = FontWeights.Bold;
Foo.Inlines.Add(block);

// Regular text.
block = new Run();
block.Text = "o o";
Foo.Inlines.Add(block);

// Bold and italics text.
block = new Run();
block.FontStyle = FontStyle.Italic;
block.FontWeight = FontWeights.Bold;
block.Text = "dd";
Foo.Inlines.Add(block);